6lO THE NERVOUS SYSTEM 



of the medulla oblongata. Traced into the substance of the brain and 

 medulla, the roots of the nerves are found to take origin from various masses 

 of gray matter. 



The roots of the first or olfactory and of the second or optic nerves are 

 discussed elsewhere. The third and fourth nerves arise from gray matter 

 beneath the corpora quadrigemina; and the roots of origin of the remainder 

 of the cranial nerves can be traced to gray matter in the floor of the fourth 

 ventricle, and in the more central part of the medulla, around its central 

 canal, as low down as the decussation of the pyramids. 



According to their several functions the cranial nerves may be thus 

 arranged: 



C Olfactory, optic, auditory, parts of 



Nerves of special sense -j the facial, glosso-pharyngeal, and of 



(__ the trigeminal. 



-vr r ( The greater portion of the trigeminal, 



Nerves of common sensation 4 . F , . . , 



( and part of the facial. 



( The motor oculi, trochlearis, lesser di- 

 Nerves of motion -j vision of the tri-geminal, abducens, 



(_ hypoglossal, and spinal accessory. 

 Mixed nerves... , Facial, glosso-pharyngeal, and vagus. 



The physiology of the first, second, and eighth nerves will be considered 

 with the Organs of Special Sense, see also figure 416. 



The Third Nerve or Motor Oculi. Origin. The third nerve arises in 

 three distinct bands of fibers from the gray nuclei surrounding the aqueduct 

 of Sylvius near the middle line, but ventral to the canal, figure 408. The 

 nucleus of origin consists of large multipolar ganglion cells, and extends to 

 the back part of the third ventricle as far as the level of the superior cor- 

 pora quadrigemina. The fibers pass from their origin partly through the 

 red nucleus to their superficial origin in front of the pons at the median side 

 of each crus. The third nerve does not decussate. 



Function. The third nerve supplies the levator palpebrae superioris mus- 

 cle, and all of the muscles of the eyeball, except the superior oblique, to 

 which the fourth nerve is distributed, and the rectus externus which re- 

 ceives the sixth nerve. Through the medium of the ophthalmic or lenticular 

 ganglion, of which it forms what is called the short root, it also supplies 

 motor filaments to the iris and ciliary muscle. The fibers which subserve 

 the three functions, accommodation, contraction of the pupil, and nerve- 

 supply to the external ocular muscles, arise from three distinct groups of 

 cells. Optic reflexes involving movements of the eyeballs are mediated 

 through fibers from cells of the superior corpora quadrigemina (which re- 

 ceive fibers from the optic nerve). These fibers from the corpora quadri- 

 gemina descend, chiefly through the posterior longitudinal bundle, figure 



